4.1 Extreme temperatures: heat waves and cold waves

Extreme temperatures pose serious risks to health, infrastructure, and ecosystems. Heat waves and cold waves can kill, overwhelm power grids, and destroy crops. Understanding what drives these events and how to prepare for them is central to disaster risk reduction, especially as climate change makes both types of extreme more frequent and intense.

Impacts of Heat and Cold Waves

Health and Safety Risks

Heat and cold extremes affect the body in opposite ways, but both can be deadly. Heat waves cause a spectrum of illness ranging from heat cramps to heat exhaustion to heat stroke, which is a medical emergency where the body's core temperature exceeds 40°C (104°F) and organ damage begins. Cold waves bring hypothermia (dangerously low core body temperature) and frostbite (tissue damage from freezing), both of which can be fatal without treatment.

• Vulnerable populations bear the greatest burden: the elderly, young children, outdoor workers, people experiencing homelessness, and low-income communities with limited access to cooling or heating.
• Extreme temperatures worsen pre-existing conditions. Heat stresses the cardiovascular system, while cold air can trigger asthma attacks and increase heart attack risk.
• The 2003 European heat wave killed an estimated 70,000 people, illustrating how deadly sustained high temperatures can be even in wealthy nations.

Infrastructure and Energy Impacts

Extreme temperatures push infrastructure beyond its design limits. During heat waves, demand for air conditioning can spike so sharply that power grids experience blackouts or brownouts. During cold waves, heating demand surges in the same way. The 2021 Texas winter storm, for example, caused widespread power failures that left millions without heat for days.

• Heat causes roads and rail lines to buckle. Steel rails can expand and warp at temperatures above about 49°C (120°F) at the rail surface.
• Cold causes water pipes to freeze and burst, and repeated freeze-thaw cycles crack pavement and bridge decks.
• Higher energy consumption during these events drives up costs for consumers and increases greenhouse gas emissions from fossil-fuel power plants.

Environmental and Agricultural Consequences

• Heat waves can trigger crop failures and wildfires. The 2010 Russian heat wave destroyed roughly 25% of the country's grain crop and caused massive wildfires around Moscow.
• Cold waves cause frost damage to crops and can kill wildlife outright, particularly species that lack behavioral or physiological adaptations to sudden cold snaps.
• Extreme temperatures alter phenology, the timing of biological events like flowering, migration, and hibernation. When these cycles fall out of sync (for instance, pollinators emerging before the flowers they depend on), entire food webs can be disrupted.
• Prolonged or repeated extremes gradually shift ecosystem composition, favoring heat-tolerant or cold-tolerant species and reducing biodiversity over time.

Factors Contributing to Extreme Temperatures

Atmospheric Circulation Patterns

Most extreme temperature events trace back to large-scale patterns in the atmosphere.

• Heat waves typically form under persistent high-pressure systems (sometimes called "heat domes"). These systems push air downward, compressing and warming it while suppressing cloud formation and rain. Without clouds, solar radiation heats the surface unchecked.
• Cold waves happen when frigid Arctic air masses are displaced toward lower latitudes. This often occurs when the polar jet stream develops large, slow-moving waves that dip far south, pulling Arctic air with them.
• Blocking patterns occur when high-pressure ridges stall in place for days or weeks, locking either hot or cold conditions over a region and preventing normal weather progression.
• Large-scale climate oscillations called teleconnections also play a role. El Niño events, for instance, redistribute heat across the Pacific and can shift temperature extremes globally. The North Atlantic Oscillation influences winter cold wave frequency across Europe and eastern North America.

Urban Heat Island Effect

Cities are consistently warmer than surrounding rural areas, a phenomenon known as the urban heat island (UHI) effect. During heat waves, this temperature difference can add 5–10°C on top of already dangerous conditions.

Several factors drive UHI:

• Dark surfaces like asphalt and concrete absorb and re-radiate far more heat than vegetation or soil.
• Cities have less vegetation, which means less evapotranspiration, the process by which plants release water vapor and cool the air.
• Tall buildings and narrow streets create "urban canyons" that trap heat and block wind, reducing nighttime cooling.
• Anthropogenic heat from vehicles, air conditioners, and industrial activity adds warmth directly to the urban atmosphere.

Antecedent Conditions and Feedback Loops

The conditions leading up to an extreme temperature event often determine how severe it becomes.

• Drought before a heat wave is a dangerous combination. Dry soil can't cool the air through evaporation, so more solar energy goes directly into heating the surface. This is why some of the worst heat waves occur in regions already experiencing drought.
• Wet conditions before a cold wave increase the risk of ice storms and heavy snow accumulation, compounding cold-related hazards.
• Positive feedback loops amplify extremes. The snow-albedo feedback is a clear example: as snow melts during a warm spell, darker ground is exposed, which absorbs more heat, which melts more snow. In reverse, fresh snowfall increases reflectivity and can deepen cold.
• Land cover changes like deforestation and urbanization alter how the surface absorbs and releases energy, shifting local and regional temperature patterns.

Mitigating Extreme Temperature Risks

Early Warning Systems and Public Health Measures

Preparation saves lives. Effective response to extreme temperatures follows a sequence:

1. Forecast and monitor using weather models and real-time temperature data.
2. Assess risk by identifying which populations and areas are most vulnerable.
3. Communicate warnings through media, emergency alerts, and community networks, with enough lead time for people to act.
4. Activate emergency resources such as cooling centers (during heat waves) or warming shelters (during cold waves) where vulnerable people can find refuge.

Beyond emergency response:

• Distributing fans, portable air conditioners, or blankets to at-risk individuals prevents illness and death.
• Public education campaigns should focus on practical actions: staying hydrated, wearing appropriate clothing, avoiding peak heat hours, and checking on neighbors who live alone.

Infrastructure Adaptations and Resilience

• Reflective ("cool") roofs and green roofs reduce how much heat buildings absorb. A white roof can be 30°C cooler in direct sunlight than a dark one.
• Improved insulation and upgraded HVAC systems help buildings maintain safe temperatures with less energy.
• Urban greening, including street trees, parks, and green corridors, directly counteracts the urban heat island effect. A single large tree can provide the cooling equivalent of about 10 room-sized air conditioners.
• Power grid resilience matters enormously. Backup generation, grid interconnections, and demand-response programs help prevent the cascading blackouts that turn extreme temperatures into full-scale disasters.

Sustainable Urban Planning and Design

Long-term resilience requires designing cities with extreme temperatures in mind.

• Passive cooling techniques like natural ventilation, strategic shading, and building orientation reduce energy demand without mechanical systems.
• Mixed land-use and compact development patterns reduce the total area of heat-absorbing surfaces compared to sprawling, car-dependent layouts.
• Preserving and expanding green spaces (urban forests, wetlands, riparian corridors) provides temperature regulation along with other ecosystem services like stormwater management.
• Cool pavement technologies, including reflective coatings and permeable materials, lower surface temperatures and reduce heat radiated back into the air.

Climate Change and Extreme Temperatures

Increasing Frequency, Intensity, and Duration

Climate change is shifting the baseline. As average global temperatures rise, the probability distribution of temperature shifts toward the hot end, meaning what used to be a rare heat wave now occurs more often and reaches higher peaks.

• The IPCC reports that heat waves that previously occurred once every 50 years are now roughly 5 times more likely than they were in the pre-industrial era.
• Arctic warming (occurring at roughly 2–4 times the global average rate) disrupts the jet stream, which can paradoxically lead to more frequent cold outbreaks at mid-latitudes as polar air spills southward.
• The warm season is lengthening in many regions, extending the window during which heat waves can form.
• Nighttime temperatures are rising faster than daytime temperatures. This is critical because the body relies on cooler nights to recover from daytime heat stress. When nights stay warm, heat-related mortality increases sharply.

Compounding Risks and Cascading Impacts

Extreme temperatures rarely act in isolation. They interact with other hazards to create compounding crises.

• Heat waves worsen drought, increase wildfire risk, and degrade air quality (ground-level ozone formation accelerates in heat). A population dealing with wildfire smoke and extreme heat simultaneously faces far greater health risks than from either hazard alone.
• Cascading impacts cross sectors: a heat wave can reduce crop yields, strain the power grid, slow transportation (rail buckling, flight cancellations), and overwhelm hospitals, all at the same time.
• Low-income and marginalized communities face disproportionate exposure (less access to cooling, more likely to live in heat island zones) and have fewer resources to recover, deepening existing social inequalities.
• Ecosystem disruption from extreme temperatures has downstream economic consequences, including reduced crop pollination from declining insect populations and weakened carbon sequestration from stressed forests.

Projections and Adaptation Imperatives

• Climate models consistently project more record-breaking heat waves through the 21st century, with the most severe increases in tropical and subtropical regions.
• Cold waves may become less frequent overall in a warming world, but they won't disappear. Jet stream disruptions can still deliver dangerous cold snaps, and communities that experience cold less often may be less prepared when it arrives.
• Effective adaptation requires both mitigation (reducing greenhouse gas emissions to slow warming) and adaptation (adjusting systems and behaviors to cope with changes already underway).
• Building resilience means improving infrastructure, strengthening social safety nets for vulnerable populations, and fostering community-level preparedness.
• Climate projections need to be integrated into urban planning, building codes, and emergency management so that decisions made today account for the conditions of the coming decades.